1. Field of the Invention
Applicant's invention relates to a purified natural zeolite pigment that can be used as a microparticle retention aid that produces a paper that exhibits improved characteristics over existing papers made with other retention aids.
2. Background Information
Paper is a complex composite made up of a combination of biological, synthetic, and inorganic materials. The components include wood pulp or other fibers and fines (as well as other components of wood), inorganic (mineral) and organic fillers, natural and synthetic polymers (for sizing, retention and strength), and other additives to meet specific product or process requirements. Retention of the individual components in appropriate amounts is critical to the properties and quality of the paper sheet as well as minimizing pollution and cost.
Retention has been defined in the literature as the term used to describe the effectiveness of a given process to retain the components of the paper sheet or to describe the ability of a given material to be retained.1 Retention describes the amount of a given material in the final product relative to the amount present at some earlier stage in the process. 1 Scott, W. E., Principles of Wet End Chemistry TAPPI Press: Atlanta (1996), p. 111.
In the past decade, retention has gained even more importance due to many changes in the paper industry. Paper machines have become bigger and run faster. Most fine paper mills have converted to alkaline papermaking conditions. This has permitted the use of new and less expensive filler systems, predominately calcium carbonate in some form (precipitated, ground, or chalk). In addition to a cost advantage, these fillers impart properties needed to meet more stringent product requirements. For example, very often the same sheet is expected to be suitable for both ink-jet printing and xerography. Generally the filler content of the sheet has increased and is likely to continue to increase. The switch to alkaline papermaking conditions has also resulted in a change in sizing chemistries. Synthetic sizes such as ASA (alkenyl succinic anhydride) and AKD (alkyl ketene dimer) are the predominant sizes used in alkaline papermaking.2 How they interact with other components of the sheet and how and where they are retained is critical to the properties of the sheet. There are now trends toward neutral or alkaline conditions and increased filler usage in wood containing grades also. AS paper manufacturers recognize the costs of poor retention in terms of pollution abatement and product loss, they are striving to reduce or eliminate effluents from their mills. All these factors combine to make retention of papermaking materials one of the most important processes of the wet end operation.3 2 Gess, J. M., Tappi Journal 75 (4): 80 (1992).3 Doiron, B. E., 1994 TAPPI Papermakers Conference Proceedings, TAPPI Press: Atlanta (1994), p. 603.
Retention of the various components of the stock in the final sheet is generally considered to be due to chemical, mechanical, or a combination of both mechanisms. While the dissolved materials are retained by adsorption or chemical bonding to the suspended solids, the suspended solids are retained by mechanical filtration or entrapment with the forming web of fiber, or preferably by physico-chemical attachment to the fibers, which are much larger, or to one another. This will occur to some degree regardless of attractive or repulsive forces between the particles. Because of their relatively small size, the particles which make up the fines fraction (inorganic fillers and cellulosic fines) are difficult to retain in the web, and much more of this material would pass through the wire and end up in the white water system if it were not for the addition of retention aids which enhance the colloidal retention of the fines fraction. Retention aids are water-soluble polyelectrolytes which cause the fines fraction to flocculate either with themselves or by adsorption onto the long fiber portion of the furnish, thus bringing about greater retention by both chemical and mechanical means.
Theory says there are two ways in which the fine particles in a papermaking web can be retained through physicochemical mechanisms:                1. By gathering the fine particles into a macroparticle.        2. By attaching the fine particles to the large fibers that are in turn retained at a 100% level.        
As a rule, agglomeration, flocculation or coagulation is accomplished by changing the charge of one particle in relation to another. This is done by adding a high cationic charge density, low molecular weight polymer (in the case of an acid papermaking system) to a papermaking furnish. It is expected that the fiber fines and small filler particles, because of their higher surface area in comparison to fibers, interact preferentially with the polymers. The high charge density of these polymers will cause the formation of cationic spots on the filler particles and fiber fines. It then is hypothesized that the cationic centers on the filler particles and fiber fines will be attracted to the anionic centers on the fibers, and this will result in the retention of the fines and small filler particles with the fibers. Too high of a dose of agglomerant or coagulant will result in fiber-fiber repulsion and a loss in retention.
The terms agglomeration, flocculation and coagulation are often used interchangeably in papermaking. Agglomeration or flocculation was used by those working directly with paper machine personnel, while coagulation was used by those personnel working in water treatment. Agglomeration or flocculation is that interaction that occurs between oppositely charged materials. Coagulation, on a purely theoretical level, tends to be formation of macroparticles that occurs when the zeta potential of a system approaches zero and there is a maximum physicochemical interaction between the elements of the furnish.
Microparticle retention systems are considered to influence fine particle retention through a physicochemical mechanism of coagulation. Such a mechanism has long been thought to have the greatest impact on small particle retention.4 4 Unbehend, J. E, Tappi 59 (10): 74 (1976).
Modern microparticle systems include both soluble polyelectrolytes and a very small (5–10 nm) highly charged “microparticle” to destabilize a given colloidal particle suspension through a complex mechanism. Usually inorganic in nature, these particles typically possess a large anionic surface charge. Used in combination with soluble polyelectrolytes, such as cationic starch or polyacrylamides, wither cationic or anionic, microparticle retention systems provide a very powerful tool for optimizing retention.
Colloidal silica is the predominant microparticle used in papermaking retention systems today. The original colloidal silica micro particle introduced to the paper industry was a stable colloidal dispersion of spherical amorphous silica particles, about 5 nm in size.5 A variety of particle sizes and three-dimensional silica sol structures have been presented in the last ten years.6 Some of the three-dimensional silica aggregate structures have overall aggregate size small enough (20–50 nm) to maintain the colloidal dispersion properties of the individual silica particle. 5, 7 5 Sunden, O., Batelson, P. G., Johansson, H. E., Larsson, H. M., and Svenging, P. J., U.S. Pat. No. 4,388,150 (Jun. 14, 1983).6 Johansson, H., International Patent WO 95/23021 (Aug. 31, 1995).7 Moffett, R. H., Tappi Journal 77 (12): 133 (1994).
One of the silica aggregates has been developed specifically to work with high-charged cationic polyacrylamide. This product is a highly branched, three-dimensional, silica aggregate with an overall particle size of approximately 50-nm.7 Moffett reported that the highly structured, larger sized silica aggregates appear to be the most efficient silica particles used in conjunction with a wide range of cationic polyacrylamides.7 7 Moffett, R. H., Tappi Journal 77 (12): 133 (1994).
It can be seen that one of the shortcomings of silica microparticle systems is the need to use different physical structures for the various papermaking applications. Another limitation on the use of silica microparticle retention aids is their very high cost.
Colloidal bentonite clay with a high smectite component, specifically montmorillonite, is another mineral commonly used in microparticle retention systems. The attribute similar to the silica microparticles is the high surface area and high charge on the particle, which, in combination, promotes the coagulation mechanism of retention of small fillers and fines. Colloidal bentonites that are effective in microparticle systems are three-dimensional particles that are up to 300 nm long and have a very thin, uniform thickness of less than 1 nm.8 High purity montmorillonite is critical for using colloidal bentonites as a microparticle in retention systems.8 8 Kundson, M. I., 1993 TAPPI Papermakers Conference Proceedings, TAPPI Press: Atlanta, 1993, p. 141.
Other types of inorganic microparticle retention systems have been presented in the literature.9,10,11 The filler retention performance of the system based on aluminum hydroxide in-situ in conjunction with cationic starch is close to that of silica and bentonite-based microparticle systems. From an economic standpoint, the level of cationic starch needed results in an expensive system and can result in paper quality problems, such as poor sheet formation. Additionally, because of the unique pH-dependent distribution of alumina species, fines retention is very dependent upon pH. While good retention performance can be obtained in a pH range from 7.8–8.6, a pH drop to only 7.5 can result in a 25% reduction in fines retention.12 9 Bixler, H. J. and Peats, S., U.S. Pat. No. 5,071,512 (Dec. 10, 1990)10 Jokinen, O. J. Petander, L. and Virta, P. J., U.S. Pat. No. 4,756,801 (Jul. 12, 1988).11 Gill, R. A. and Sanders, U.S. Pat. No. 4,892,590 (Jan. 9, 1990).12 Gill, R. I. S., Paper Tech., 32(8): 34 (1991).
Existing microparticulate retention aids, namely silica and bentonite, have many disadvantages, so a goal of the present invention was to develop a microparticle retention system that incorporates a zeolite pigment with at least the same or superior qualities to those of the existing microparticles.
A zeolite pigment that possesses the desirable combination of brightness, color, particle size distribution, surface area, internal void volume, rheology and hardness could also be useful in overcoming the limitations of conventional and other specialty pigments in various papermaking and paper coating applications including but not limited to: (1) more economical microparticle retention system chemistry; (2) toner bond improvement in laser and other dry toner imaged digital papers; (3) elimination of smudging and improvement of print quality in direct print flexography on coated linerboard used in corrugated containers; (4) elimination of print through on newsprint and ultra light weight coated papers; (5) improvement of dot fidelity and print quality on coated rotogravure printing papers; (6) low abrasion extender for titanium dioxide pigments; (7) improvement of coefficient of friction of paper and paperboard; (8) production of technical specialty papers such as anti-tarnish, gas filtration, and absorbent papers with improved properties and lower cost of manufacture; (9) additive to improve the efficiency of deinking systems; (10) additive to reduce problems with pitch, stickies and/or other organic deposits in pulping and papermaking systems.
Zeolites are crystalline, hydrated aluminosilicates of the alkali and alkaline earth metals. More particularly, zeolites are framework silicates consisting of interlocking tetrahedrons of SiO4 and AlO4. In order to constitute a zeolite, the ratio of silicon and aluminum to oxygen must be 2. The aluminosilicates structure is negatively charged and attracts the positive cations that reside within. When exposed to higher charged ions of a new element, zeolites will exchange the lower charged element contained within the zeolite for a higher charged element. Unlike most other tectosilicates, zeolites have large vacant spaces or cages in their structures that allow space for large cations such as sodium, potassium, barium, and calcium and relatively large molecules and cationic molecules, such as water, ammonia, carbonate ions, and nitrate ions. In most useful zeolites, the spaces are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow ease of movement of the resident ions and molecules into and out of the structure.
Zeolites are characterized by 1) a high degree of hydration, 2) low density and large void volume when dehydrated, 3) stability of the crystal structure of many zeolites when dehydrated, 4) uniform molecular sized channels in the dehydrated crystals, 5) ability to absorb gases and vapors, 6) catalytic properties, and 7) cation exchange properties.
There are several mentions of the use of synthetic zeolites as a wet end additive in papermaking. In U.S. Pat. No. 4,752,314 Rock teaches the use of a combination of titanium dioxide and synthetic Zeolite A wherein the sodium has been at least partially replaced with calcium and/or hydronium ion to improve the optical properties of paper. Rock teaches that the Zeolite A must have a composition: Zeolite (Ca.sub.x Na.sub.y)A zH.sub.2 O where x is in the range of 0.3 to 3.6, y is in the range of 9.6 to 11.85 and z is in the range of 20 to 27 or Zeolite (Ca.sub.x Na.sub.y Hy) zH.sub.2 O where x is in the range of 0 to 4.8, y is in the range of 0.6 and z is in the range of 20 to 27.
In U.S. Pat. No. 5,900,116 Nagan teaches the use of a synthetic zeolite crystalloid coagulant with particle size 4 to 10 nm in combination with cationic acrylamide polymer as a papermaking retention aid.
The use of natural zeolites in paper making has a long history, but has been almost unique to Japan where zeolite has been used as filler to improve bulkiness and printability.13 Natural zeolites have also been used as fillers for paper in Hungary. These natural zeolites however are a low brightness material and this renders it unsatisfactory for application in the United States on uncoated office paper and on coated ink jet paper where high brightness is expected. 13 Japanese patent application No. 45-41044 with disclosure date Dec. 23, 1970.
Numerous families of natural zeolites exist and each has varying characteristics. Unfortunately, natural zeolites exhibit nonuniform properties that make them difficult to work with in many applications because ores from one location can vary with any other. It is however possible to manufacture zeolites with uniform properties. The preferred zeolite for use in the present invention is a processed form of the natural mineral clinoptilolite which is a hydrated sodium potassium calcium aluminum silicate having the formula (Na, K, Ca)2-3 Al3 (Al,Si)2 Si13)36-12H2O. This zeolite is within the family Heulandite that also includes the mineral heulandite, which is a hydrated sodium calcium aluminum silicate. The physical characteristics of raw clinoptilolite are listed in Table 1.
TABLE 1PHYSICAL CHARACTERISTICS OF CLINOPTILOLITEColor is colorless, white, pink, yellow, reddish and pale brown.Luster is vitreous to pearly on the most prominent pinacoid face and oncleavage surfaces.Transparency: Crystals are transparent to translucent.Crystal System is monoclinic; 2/m.Crystal Habits include blocky or tabular crystals with good monocliniccrystal form. More tabular and proportioned than heulandite.Also commonly found in acicular (needle thin) crystal sprays.Cleavage is perfect in one direction parallel to theprominent pinacoid face.Fracture is uneven.Hardness is 3.5 B 4, maybe softer on cleavage surfaces.Specific Gravity is approximately 2.2Streak is white.
Clinoptilolite's structure is sheet like with a tectosilicate structure where every oxygen is connected to either a silicon or an aluminum ion (at a ratio of [Al+Si]/0=2). The sheets are connected to each other by a few bonds that are relatively widely separated from each other. The sheets contain open rings of alternating eight and ten sides. These rings stack together from sheet to sheet to form channels throughout the crystal structure. The size of these channels controls the size of the molecules or ions that can pass through them. Clinoptilolite is well suited for various applications, such as in paper coating compositions, because it exhibits large pore space, high resistance to extreme temperatures, and has a chemically neutral structure.
The zeolite of the present invention is not anticipated by either Rock in U.S. Pat. No. 4,752,341 or Nagan in U.S. Pat. No. 5,900,116. The structure of the natural zeolite of the present invention falls outside of the range of structures specified by Rock in U.S. Pat. No. 4,752,341. The particle sizes of the natural zeolite of the present invention are 2 to 3 orders of magnitude greater than the 4 to 10 nm specified by Nagan in U.S. Pat. No. 5,900,116.